1. Field of the Invention
The present invention relates to a method and an apparatus for detecting maximum and minimum values of two detection signals with different phases which are outputted from a rotational angle detecting device provided in a torque detecting apparatus and a steering apparatus, and a torque detecting apparatus and a steering apparatus comprising the maximum and minimum values detecting apparatus.
2. Description of Related Art
Some steering apparatuses for vehicles assist steering by driving an electric motor so as to reduce driver's load. Such a steering apparatus comprises: an input shaft joined to a steering wheel; an output shaft joined to tire wheels via a pinion and a rack, for example; a connecting shaft for connecting the input shaft and the output shaft; a torque detecting apparatus for detecting a steering torque applied to the input shaft based on a torsional angle generated on the connecting shaft; and a steering assist electric motor interlocked with the output shaft, which is driven in a controlled manner based on the steering torque detected by the torque detecting apparatus.
FIG. 1 is a schematic representation which schematically shows a constitution of a rotational angle detecting device and a torque detecting apparatus proposed by the present applicant in Japanese Patent Application Laid-Open No. 2001-324321. FIG. 2 is a vertical sectional view showing a constitution of a main part of a steering apparatus comprising the rotational angle detecting device and the torque detecting apparatus.
An input shaft 16 is connected to a steering wheel 1 at an upper end thereof. An output shaft 17 is connected to a pinion 18 of a steering mechanism at a lower end thereof. The input shaft 16 and the output shaft 17 are coaxially connected to each other via a torsion bar 19 (a connecting shaft) of a small diameter, to constitute a steering shaft 13 which connects the steering wheel 1 and the steering mechanism. The input shaft 16 and output shaft 17 have portions proximal to a connection portion which are constituted as described below. The input shaft 16 and output shaft 17 are supported in a housing 23 via bearings 21 and 22, so as to be rotatable respectively. The housing 23 is fixed at an unshakable portion of a vehicle, with a mounting bracket 24.
The housing 23 houses a sensor box 11 of a torque detecting apparatus, which will be described below, and a reduction mechanism 25 for reducing rotation of a steering assist electric motor 26, which is driven based on a result detected by the torque detecting apparatus, and transmitting the reduced rotation to the output shaft 17. The electric motor 26 is rotated so as to assist operations of the steering mechanism in accordance with rotation of the steering wheel 1, so that driver's load for steering can be reduced. A lower end portion of the output shaft 17 is connected to a steering mechanism of a rack-and-pinion type via a universal joint.
A disc-shaped target plate 12 (a rotational member) is coaxially fixed at a peripheral surface of a portion of the input shaft 16, which is proximal to an end portion connected to the output shaft 17. The target plate 12 has a plurality of (five in FIG. 1) targets 15 which are juxtaposed on a peripheral surface of the target plate 12.
Each target 15 is a protrusion made of a magnetic material which comprises a first inclined portion 15a which is inclined to one direction and a second inclined portion 15b which is inclined to another direction along a peripheral surface of the target plate 12, as shown in a development elevation in FIG. 3 which shows the peripheral surface of the target plate 12 in a developed form. The targets 15 are juxtaposed on the peripheral surface of the target plate 12 at regular intervals in a peripheral direction.
The first inclined portion 15a and second inclined portion 15b are provided approximately symmetrically with respect to a line which passes a connection point of the portions 15a and 15b and is parallel to a rotational axis of the target plate 12.
Another target plate 12, which comprises the same targets as those described above, is fixed at a peripheral surface of a portion of the output shaft 17, which is proximal to an end portion connected to the input shaft 16. Each target 15 provided on the target plate 12 of the output shaft 17 and each target 15 provided on the target plate 12 of the input shaft 16 are aligned in a peripheral direction.
The sensor box 11 is provided outside the target plates 12, to face the periphery of targets 15 on the peripheral surface of the target plates 12. The sensor box 11 is fixedly supported at the housing 23 which supports the input shaft 16 and output shaft 17 so as to be rotatable. The sensor box 11 houses MR sensors 1A and 1B (first detecting means and second detecting means) which oppose two different portions in a peripheral direction of the targets 15 of the input shaft 16, and MR sensors 2A and 2B (first detecting means and second detecting means) which oppose two different portions in a peripheral direction of the targets 15 of the output shaft 17. The MR sensors 1A and 1B and the MR sensors 2A and 2B are housed with the peripheral positions thereof aligned accurately.
Each of the MR sensors 1A, 2A, 1B and 2B is composed of an element, such as a magneto-resistance effect element (an MR element), having an electric characteristic (a resistance) which changes by the action of a magnetic field, so that a detection signal changes in accordance with a proximal portion of the target 15 which the MR sensor faces. Detection signals from the MR sensors 1A, 2A, 1B and 2B are supplied to a signal processing unit 14 which uses a microprocessor, which is provided outside or inside the sensor box 11.
The following description will explain in brief the operations of the rotational angle detecting device and torque detecting apparatus having such a constitution.
As described above, each of the targets 15, which the MR sensors 1A, 2A, 1B and 2B face, is a protrusion made of a magnetic material which comprises a first inclined portion 15a which is inclined to one direction and a second inclined portion 15b which is inclined to another direction along each peripheral surface of each target plate 12 coaxially fixed at each of the peripheral surfaces of the input shaft 16 and output shaft 17, the first and second inclined portions 15a and 15b being juxtaposed at regular intervals in a peripheral direction.
Consequently, when the input shaft 16 (or the output shaft 17) is rotated on an axis, each of the MR sensors 1A and 1B (or 2A and 2B) outputs detection signals (referred to as “A” and “B”) which rise and fall proportionally in accordance with change of the rotational angle of the input shaft 16 (or the output shaft 17), as shown in FIG. 4, while the corresponding targets 15 pass a position opposing each MR sensor.
The detection signals of the MR sensors 1A and 1B correspond to the rotational angle of the input shaft 16 which is provided with the targets 15 corresponding to the MR sensors 1A and 1B. The detection signals of the MR sensors 2A and 2B correspond to the rotational angle of the output shaft 17 which is provided with the targets 15 corresponding to the MR sensors 2A and 2B.
As a result, the signal processing unit 14 can calculate the rotational angle of the input shaft 16 based on the detection signals of the MR sensors 1A and 1B, i.e., the signal processing unit 14 and the MR sensors 1A and 1B operate as a rotational angle detecting device for the input shaft 16. Meanwhile, the signal processing unit 14 can calculate the rotational angle of the output shaft 17 based on the detection signals of the MR sensors 2A and 2B, i.e., the signal processing unit 14 and the MR sensors 2A and 2B operate as a rotational angle detecting device for the output shaft 17.
When a rotational torque is applied to the input shaft 16, there arises a difference between each of the detection signals of the MR sensors 1A and 1B and each of the detection signals of the MR sensors 2A and 2B.
The phase of the MR sensors 1A, 2A and the phase of the MR sensors 1B, 2B are different from each other in a peripheral direction of the target plates 12, at an electric angle of 90° for example. Consequently, the detection signals can compensate a non-linearly changing range mutually. The phase angle may be any electric angle of 1° through 360° if the compensation is possible.
The difference between the detection signal of the MR sensor 1A and the detection signal of the MR sensor 2A, or the difference between the detection signal of the MR sensor 1B and the detection signal of the MR sensor 2B, corresponds to a difference (a relative angular displacement) of the rotational angle of the input shaft 16 and the rotational angle of the output shaft 17. The relative angular displacement corresponds to a torsional angle which arises on the torsion bar 19 connecting the input shaft 16 and the output shaft 17 by the action of the rotational torque applied to the input shaft 16. Consequently, the rotational torque applied to the input shaft 16 can be calculated based on the difference of the detection signals described above.
Each individual MR sensor 1A, 2A, 1B or 2B has a peculiar output characteristic. Consequently, there arises an error in an amplitude of the detection signals of the MR sensors 1A, 2A, 1B and 2B caused by difference in gain characteristics shown in FIG. SA, and there arises an error in a median of the detection signals caused by difference in offsets shown in FIG. 5B. When the detection signals include an error, a torque cannot be calculated accurately. To calculate the rotational torque with high accuracy, it is required to correct medians Amid and Bmid and amplitudes App and Bpp of the detection signals A and B to be values (a reference median Vmid0 and a reference amplitude Vpp0) which were set in designing the torque detecting apparatus.
One method for correcting the detection signals is a method to store in advance a translation table in the signal processing unit 14, which translation table, for example, includes signal values to be outputted from detection signals A and B and corresponding corrected signal values, when manufacturing and assembling the torque detecting apparatus. By referring to the translation table, the signal processing unit 14 can output corrected detection signals corresponding to the detection signals outputted from the MR sensors 1A, 2A, 1B and 2B.
However, to make the translation table, it is necessary to survey the output characteristics of all the MR sensors. Moreover, it is also necessary to add an area for storing the translation table in the signal processing unit 14. The above method demands much labor and high costs for making the translation table.
Another correcting method is a method to detect maximum values Amax and Bmax and minimum values Amin and Bmin of the detection signals A and B by the signal processing unit 14, to calculate amplitudes of the detection signals A and B with the formulas:App=Amax−AminBpp=Bmax−Bminto calculate medians of the detection signals A and B with the formulas:Amid=(Amax+Amin)/2Bmid=(Bmax+Bmin)/2and to correct the amplitudes and medians of the detection signals A and B to be the reference amplitude Vpp0 and the reference median Vmid0.
Maximum and minimum values of the detection signals A and B can be calculated based on maximum and minimum values of signal values detected within a period. However, a period of the detection signals A and B changes in accordance with a rotational speed of the steering wheel 1 which is manipulated by a man. It is hard to detect maximum and minimum values when a period is not constant.
A proposed method for detecting maximum and minimum values of the signals A and B having a changing period is a method to set an upper bound threshold and a lower bound threshold as shown in FIG. 4, to detect passing of a maximum value and a minimum value when the upper and lower bound thresholds and the detection signals A and B cross at a predetermined angle, and to detect maximum and minimum values based on signal values detected within a time range when the passing was detected.
However, in the above method, it is necessary to preset thresholds. Moreover, it is impossible to detect passing of a maximum value or a minimum value when the detection signals A and B do not come to the upper bound threshold or the lower bound threshold.